1. Field of the Invention
The present invention relates to a sheet feeding device for moving a sheet such as a paper sheet and, more particularly, to a sheet feeding device suitable as a paper sheet feeding device used in equipment such as a calculator, a copying machine, a facsimile apparatus, a wordprocessor, and a typewriter.
2. Related Background Art
A conventional sheet feeding device for clamping a sheet such as a paper sheet between a pair of vibration members such as piezoelectric elements to generate travelling vibrations and for feeding the sheet is proposed by the present assignee. A prior-art invention of a sheet feeding device proposed by the present assignee is disclosed in Japanese Patent Laid-Open No. 59-177243.
FIG. 2 is a view showing an arrangement of the sheet feeding device disclosed in Japanese Patent Laid-Open No. 59-177243.
Referring to FIG. 2, vibration elements 3 and 4 such as piezoelectric or electrostrictive elements are fixed on a vibration member 1, and a similar vibration element 5 (although one more element is actually provided, it is not illustrated in FIG. 2) is fixed to a vibration member 2. A supporting member 6 supports the vibration member 1 and biases the vibration member 1 against a sheet S. A housing base plate 7 partially constitutes a housing of the sheet feeding device. The vibration member 2 is fixed on the base plate 7. Side plates 8 and 9 are fixed on the base plate 7 and the supporting member 6. In this device, an AC voltage having a frequency (a natural frequency of one of the vibration members in practice) near the natural frequencies of both the vibration members is applied to one vibration element of each of the vibration members, while an AC voltage having a phase shifted from that of the AC voltage applied to one vibration element by .pi./2 from a .pi./2 shifter is applied to the other vibration element of each vibration member, so that travelling vibration waves are generated on the vibration members symmetrically about a sheet feeding surface. Each point of the surface of each vibration member opposite to the surface having the vibration elements is subjected to a kind of elliptical motion, and the points of the opposite surfaces of the vibration members are subjected to elliptical motion symmetrically about the sheet feeding surface. Therefore, the sheet can be fed by a frictional force.
FIG. 3 is a schematic view for explaining the principle of generating a sheet feeding force upon generation of travelling vibration waves. Assume that the sheet S is clamped by the vibration members 1 and 2 at an appropriate urging force, and that travelling vibrations are generated by the vibration members 1 and 2, as shown in FIG. 3. At this time, a given mass point on the surface of each of the vibration members 1 and 2 is taken into consideration. This mass point generally performs motion which traces an elliptical track. For example, in the vibration member 1, when the travelling vibration wave propagates in the right direction indicated by an arrow , the mass point on the surface traces the elliptical track in the clockwise direction. Since the phases of the applied voltage components applied to the vibration members are controlled to have a spatial phase deference of 180.degree., the travelling vibration waves of the vibration members are symmetrical about the sheet, so that the projections of the opposite surfaces always contact each other through the sheet. A moving direction of each mass point corresponding to the projection portion of each of the vibration members 1 and 2 is opposite to a vibration propagating direction. In this case, a sheet feeding force is generated in a direction indicated by an arrow .rarw.. In recessed portions of the surfaces with respect to the sheet, a sheet feeding force in the same direction as the wave propagating direction is generated. However, this sheet feeding force is smaller than that generated by the projection portions. A total sheet feeding force acts in a direction opposite to the travelling vibration wave propagating direction.
FIG. 4 shows another conventional sheet feeding device arranged based o the sheet feeding principle described above. This device is also proposed by the present assignee. In this sheet feeding device, vibration members comprise a pair of annular vibration members 10 and 11 each having a planar shape like a running track, as shown in FIG. 4. Vibration elements 12 and 13 such as piezoelectric or electrostrictive elements are fixed on the nonopposing surfaces of the annular vibration members. The electrostrictive elements 12 and 13 are divided into two groups. An AC voltage having a phase difference of 90.degree. from that of an AC voltage applied to the one group is applied to the other group. When AC voltages having different phases are applied to the two groups of the electrostrictive elements 12 and 13, travelling bending vibrations which propagate along the circumferential direction of the annular vibration members 10 and 11 are generated by the vibration members 10 and 11, and a sheet S receives a force opposite to the travelling bending vibration propagating direction from the annular vibration members and is moved. The sheet feeding device using the annular vibration members having a shape like a running track utilizes travelling flexture motion of the linear portions of the vibration members as a sheet feeding force. In this sheet feeding device, the linear portions of the vibration members are arranged to be parallel to the sheet feeding direction. When the two linear portions of each of the annular vibration members 10 and 11 are in contact with the sheet in the sheet feeding device, the sheet receives the opposite direction forces and cannot be fed. In this device, only linear portions 10a and 11a of the vibration members 10 and 11 are kept in contact with the sheet S, and other portions including linear portions 10b and 11b have a thin wall so as to prevent them from contacting the sheet S.
In this device, when cyclic travelling vibration waves of the annular vibration members 10 and 11 in a direction indicated by an arrow f.sub.1 are generated, the sheet S is fed in a direction of an arrow f.sub.2 opposite to the propagation direction of the travelling vibration wave (i.e., the direction of the arrow f.sub.2 is a direction toward the upper surface of the drawing of FIG. 5).
Sheets having different thicknesses and different mechanical characteristics are used in various types of office equipment including the sheet feeding devices described above. For example, an overhead projector uses a resin sheet having properties greatly different from those of normal paper. Since such a resin sheet has a much smaller coefficient of surface friction than that of the normal paper, a sheet feeding device must have a function corresponding to the resin sheet in order to feed the resin sheet as in the case of normal paper.
Since the conventional sheet feeding devices are designed under the assumption that sheets to be fed have predetermined thicknesses and predetermined coefficients of friction, these devices cannot be applied to all kinds of sheets.